In recent times, numerous studies have been performed regarding the construction of chemical sensors making use of optical fibers. Typically, these sensors comprise a system including one or more optical fibers for conducting electromagnetic radiation, especially within the visible range. With these sensors is associated a chromophore, that is to say a substance having electromagnetic radiation absorption characteristics which change as a function of the chemical characteristics of the environment in which the substance is immersed. Sensors of this type are utilized, for example, for taking pH readings.
Currently, these sensors are constructed in accordance with various techniques. One of these techniques involves the application of the chromophore to a polymeric support which is then inserted into a container intended to be applied to the end of the optical fiber. The container is expediently slotted or otherwise made permeable in order to permit penetration by the liquid on which the reading is to be performed. The liquids penetrates within container and thus in contact with the chromophore disposed on the polymeric support. According to another technique, the chromophore is attached directly to the optical fiber rather than to a solid support. This second technique offers numerous advantages, including the high degree of miniaturization, the extremely compact structure, the absence of a probe in which to incorporate the external support and which is fixed to the optical fiber, and reduced response times. Nevertheless, the direct application of the chromophore to the surface of the fiber presents the disadvantage of a low sensitivity of the sensor thus obtained, due to the modest quantity of chromophore which can be immobilized on the fiber. Studies have been carried out on various systems for increasing the quantity of chromophore immobilized on the fibers; among these, it is possible to mention chemical treatments of the surface of the fibers to increase the number of sites available for the formation of the bond with the chromophore, immobilization on the surface of the fiber of a polymer in which the chromophore is incorporated, and the like.
A global view of the various technologies currently known for the construction of chemical sensors making use of optical fibers can be found in A. M. Schggi and F. Baldini "Chemical sensing with optical fibres", International Journal of Optoelectronics, 1993, Vol. 8, No. 2, pages 133-156.
Currently, new technologies are being investigated for fixing the chromophore on new glass-based supports. In F. Baldini and S. Bracci "Optical-fibres sensor by silylation techniques", Sensors and Actuators B, 11 (1993), pages 353-360, a description is for example given of a method for the application of a chromophore to supports made of controlled porosity glass (CPG: controlled-pore glasses): in this case also, a support constituted by the CPG particles, on which the chromophore is immobilized, is subsequently inserted into a cap applied to the terminal end of the optical fibers for conducting the electromagnetic radiation.